In general, the incorporation of heart rate monitors into exercise equipment has been desirable since the beginnings of the commercial exercise equipment industry. Heart rate has traditionally been used as a standard measure of physical condition and exercise intensity, and medical prescriptions often specify a target heart rate to be maintained during the prescribed exercise. However, traditional sonometric techniques require interruption of exercise, and catheter-based manometric measurements have proved to be too invasive for general use. Oxymetric measurements using earlobe clips and electrocardiographic measurements using conventional saline gel electrodes have not been well received by exercise equipment users. Other minimally invasive techniques such as wrist-strap sonography have proven to be susceptible to interference during strenuous exercise. Note that the definition of the term "invasive" differs slightly from its use in medical procedures in that methods which require earlobe clips, shaving of skin areas, the application of adhesive pads to skin, or even the placement of a strap underneath clothing are considered psychologically invasive of a user's personal space.